Optical components of an optical mouse are shown in FIG. 1, and to facilitate mass production and assembly, these components are manufactured separately. During assembly, a light source device 12 and a sensor chip 14 are soldered onto a printed circuit board (PCB) 16, and a light source mount 10 is fixed onto the PCB 16 in a tight fit to prevent swing of the light source device 12. The PCB 16 has an opening 162 to allow light to pass therethrough, and a lens 18 is placed beneath the opening 162 and fixed onto the PCB 16. By means of a recess 192 and a fool-proof structure 194, the lens 18 is in turn placed on a base 19 to align the light path with a light hole 196.
FIG. 2 depicts a light path of the optical mouse shown in FIG. 1. The light source device 12 is generally a red LED, and the light emitted from the light source device 12 is sequentially refracted and reflected by the lens 18, and then passes through the light hole 196 to impinge on a plane beneath the optical mouse. From there, the light is reflected back and passes through the lens 18 to be imaged onto the sensor chip 14. Different features and roughness of the desktop surface will result in different energy intensity of the reflected light to exhibit corresponding features in the image. When the optical mouse is moved, continuous patterns will be obtained in the sensor chip 14 which then, according to difference in positions of features of the continuous images, compares and analyzes each of the images to determine the moving direction and displacement of the optical mouse. In other words, imaging quality is a key factor that dominates performance of the optical mouse. Hence, when loose engagement or misalignment among individual parts of the optical mouse occurs, light reflected from the desktop surface will fail to follow the correct light path to travel to the sensor chip 14. Consequently, the sensor chip 14 will retrieve a drifting or blurred image, leading to incorrect recognition of the image.
As molds used by various manufacturers for producing the separate optical parts have non-uniform specifications, problems such as varied height of the mouse bases, poor close-fit and poor fixation effect among individual components often arise. Consequently, the resulting optical mice tend to become loose in the Z-axis (vertical) direction during operation, thereby causing shifting and out-of-focus of the light path.
As an effort to reduce the assembly errors during production, U.S. Pat. No. 6,462,330 to Venkat et al. suggests packaging the lens into a package of the sensor chip, U.S. Pat. No. 7,045,775 to Leong et al. suggests packaging both the lens and the LED into the package of the sensor chip, and U.S. Pat. No. 7,199,350 to Chien suggests packaging the lens, the LED and a controller chip of the optical mouse all into the package of the sensor chip. Although these arts feature a high level of integration, none of them is of a COB structure; rather, they are limited by the chip pins, and once the pin design is altered, the lead frame of the whole package must be remolded, which is labor-consuming, time-consuming and very costly.
On the other hand, U.S. Pat. Nos. 6,541,762 and 6,653,724 disclose an optical mouse implemented in a COB fashion, in which a sensor chip is soldered in form of a die onto a smaller secondary PCB, which is in turn fixed to a primary PCB of the optical mouse. However, this kind of optical mouse has a low level of integration because the PCB of the COB package must be fixed to another PCB (so it is also known as a secondary COB).